The emergence of tungsten incandescent lamp in the 19th century led the world into the era of artificial illumination. Since the 20th century, a revolutionary new light source, LED, has rapidly entered the lighting market due to its advantages of energy saving, environmental protection and long life. LED has become the leading lighting source in the future, and is widely used in commercial lighting, industrial lighting, outdoor lighting, and so on. However, in the past, LED light sources, such as plug-in LEDs, SMD LEDs, chip on board (COB), integrated high-power LED lamp beads, can only be plane light sources without optical devices such as lenses.
In 2008, Japanese Ushio Light Source introduced a bulb-type lamp using LED filaments configured with an incandescent lamp prototype. LED filament lamp truly realizes 360-degree full-angle light-emitting stereo light sources, which satisfies customers' full-view light-emitting requirements, brings unprecedented illumination and is more energy-saving. Since the Japanese Ushio light source was first introduced and mass-produced, candle lamps and bulb lamps with LED filaments as light sources have gradually been favored by more and more consumers in the market.
The existing LED filament lamp generally consists of a bulb shell, a plurality of LED filaments, a core column with a bracket, a driver, and a lamp cap. The LED filaments are arranged on the bracket of the core column to achieve 360-degree light emitting. The LED filament is generally formed by die bond of blue LED lamp beads without back plating on a substrate bar made of sapphire, transparent ceramic, fluorescent crystal, glass, or carved copper plate, and then the lamp beads are all connected in series by a gold wire, and the lighting bar is coated with a yellow phosphor so as to emit white light.
In order to realize 4π light emitting (i.e., 360-degree light emitting), the existing LED filaments mostly adopt a transparent substrate design, and the substrate is made of a transparent material such as sapphire, transparent ceramic or glass. For example, Taiwan Epistar, Zhejiang Ledison and other companies have obtained a large number of patents on transparent substrates. For example, the patent application No. WO/2012/031533 filed by Zhejiang Ledison Optoelectronic Co., Ltd. discloses an LED bulb and an LED lighting bar capable of 4π light emitting. The LED bulb includes: a LED light bulb shell; a core column with an exhaust tube and a bracket; at least one LED light emitting strip with LED chips emitting 4π light; a driver; and an electrical connector, wherein the LED light emitting strip comprises a transparent substrate and at least one series of LED chips on the transparent substrate and connected in series in such a manner that the PN junctions therein have a same direction, the LED chips having transparent chip substrates. 4π light emitting is realized by using the transparent substrate to increase the light-emitting rate.
In the patent application No. WO/2012/031533, the LED lighting bar with the 4π light-emitting LED chip is used to improve the light-emitting rate of the LED chip, so as to improve the luminous efficiency of the LED filament lamp, which is one of the research directions of the LED filament lamp. However, such LED filament lamps cannot solve another key problem of the LED filament lamp, i.e., heat dissipation. The heat of an LED lamp is mainly generated by an LED chip and a power device of a drive circuit. The LED lamps in the prior art generally use low-voltage and high-current power LEDs, one LED chip has one PN junction, and the operating current is as large as 0.35 A or even several ampere, the electric power of 1 W to several watt or more is concentrated on a chip of 1 to several square millimeters, the external quantum efficiency is only about 30%, with an energy difference between injected electrons and generated photons and an energy difference between photons generated by the PN junction and the last emitted photons, about 70% of the electric power will be converted into heat, and how to dissipate this large amount of heat has always been one of the key problems of LED filament lamps. LED is a semiconductor device, and the junction temperature of the PN junction rises, which will cause the luminous efficiency to drop rapidly, or even the PN junction to burn up. As the temperature rises, silica gel used to coat an LED chip will have a cracking problem, directly affecting the service life of the LED filament lamp.
For a single LED, when heat is concentrated in a chip of small size and cannot be effectively dissipated, the temperature of the chip increases, causing the non-uniform distribution of heat stress, and the reduction of the luminous efficiency of the chip and the lasing efficiency of a phosphor. Studies have shown that when the temperature exceeds a certain value, the failure rate of the device will rise exponentially, and the reliability of a component will decrease by 10% for every 2° C. rise. In order to ensure the life of the device, the PN junction temperature is generally required to be below 110° C. As the temperature of the PN junction rises, the light-emitting wavelength of a white LED device will be red-shifted. The statistical data show that at 100° C., the wavelength may be red-shifted by 4-9 nm, which leads to the increase of non-radiation of a YAG phosphor with the increase of temperature and the reduction of the conversion light energy, resulting in reduction of conversion efficiency, reduction of the total luminous intensity, and poor white light chroma. At around room temperature, for every 1° C. rise in temperature, the luminous intensity of LED will be reduced by about 1%.
In order to solve the heat dissipation problem, many LED bulb lamps in the prior art use metal heat sinks with heat sink fins, and there has been a lot of researches and patents on the material and shape of such heat sinks and how to increase the convective heat exchange with air. Such metal heat sinks are mainly made of aluminum alloy, which are heavy and costly, and have become one of the key factors for the high cost of the existing LED bulb lamps. In order to solve the problem of 4π light emitting, many LED filament substrates use sapphire or diamond, etc., but sapphire, diamond, etc. are very expensive, directly increasing the cost of the LED lamp.
In the above-mentioned patent application No. WO/2012/031533, it uses a gas convection heat dissipation structure to dissipate heat through the convection and heat conduction of gas in a bulb shell and then through a bulb. But in fact, such heat dissipation structure cannot effectively dissipate heat. First, it uses a transparent substrate, and the selected substrate material is made of glass, transparent ceramic or plastic, thus the heat conductivity coefficient is low. Heat generated by an LED chip needs to be conducted through a substrate to dissipate heat by gas convection. In this patent, the heat conductivity coefficient of the substrate is low, and under the action of heat insulation of phosphor glue, the heat generated by the LED chip cannot be effectively conducted by the transparent substrate and a phosphor glue layer, not to mention gas convection heat dissipation. Besides, the material of the bulb shell is also glass. In fact, by means of gas convection heat dissipation, it is also difficult to dissipate heat through the glass bulb shell, and heat dissipation through the glass bulb shell may also cause over-high temperature of the bulb shell, which brings a certain danger.
In order to solve the heat dissipation problem, heat dissipation by heat radiation is used. On Nov. 9, 2016, the applicant filed a patent application claiming for power-supply built-in LED filament lamp using heat-radiating material. By arranging a layer of heat-radiating material on the surface of the LED filament, heat is dissipated through heat radiation, but there was no systematical explanation of radiation heat dissipation, and there is room for improvement in the heat dissipation effect.
The radiation heat dissipation may be understood as the way in which an infrared ray is emitted from the surface of a higher-temperature object and is received by a lower-temperature object. As we all know, the space outside the atmosphere is close to absolute zero, and the temperature in the upper atmosphere is also quite low. This is also a natural huge cold storage. The huge capacity of the space makes it a “black hole” for heat. If we discharge the unwanted heat on the ground into the space in the form of electromagnetic waves, we can achieve the purpose of cooling. Radiation cooling is such a non-consumption mode of cooling.
The researchers analyze the spectral transmission characteristics of the Earth's atmosphere, and the transmission spectrum is shown in FIG. 1. It can be seen from FIG. 1 that the atmosphere has different transmittances for different wavelengths of radiation. In an interval where the transmittance is high, the electromagnetic waves in the wavelength band can penetrate through the atmosphere more freely, and these intervals are meteorologically referred to as the “window” of the atmosphere. The spectral transmission characteristics of the atmosphere are mainly determined by water vapor, carbon dioxide and ozone in the atmosphere. The change in their content causes a change in transmittance, but the distribution of the transmission spectrum does not change much. Among several atmospheric windows, the band of 8 to 13 μm is more notable because the wavelength of blackbody radiation at normal temperature is mainly concentrated in this band. The atmosphere is permeable to electromagnetic radiation in this band. So, if there is a material that converts heat into electromagnetic waves in this particular band, heat waste can leave the earth. As a heat-dissipating mode that does not consume energy, it will have broad application prospects in the field of LED filament lamps.
In addition to the heat dissipation problem, preventing blue light leakage is also one of the key problems in the LED industry. In recent years, a lighting technology has improved in terms of light output and lifetime, but in order to improve brightness, each novel light source has greatly increased the content of blue light, especially LED bulbs. At present, a white LED technology can be realized in a variety of ways, mainly in two ways: one is that a blue light technology coordinated with a yellow phosphor so as to form white light; the other is that multiple monochromatic lights are mixed to obtain white light. Since the driving voltage, the light output, the temperature characteristics and the lifetime of different color LEDs are different, a multi-color hybrid white LED is complicated in production and high in cost. Manufacturers generally use the first technology.
However, it may bring more harm when there is too much blue light in an LED bulb. The light source has a great influence on the physiology and behavior of a human body. Excessive blue light inhibits melatonin and increases stress hormone cortisol, which interferes with physiological mechanisms. In addition to affecting sleep, it may damage the retina and cause other diseases. In the blue light spectrum, the wavelength of 400 to 450 nm is most likely to cause retinal damage, which can penetrate through the lens of a human eye to reach the retina, and cause photochemical damage to the retina. There are even studies show the relationship of blue light to breast cancer. Japanese studies have found that all blue light radiation larger than 20 J·cm−2 causes significant fundus changes. Moreover, the lens of the human eye gradually develops with the age of the person since its formation. Therefore, the greatest risk of blue light hazards occurs in infants and young children. At this time, the underdeveloped lens of the human eye has a high transmittance for short-wavelength spectral radiation, which is several times that of an adult's eye. Short-wave blue light is more likely to reach the retina through the lens of such a person, thereby accelerating oxidation of cells in the macular area of the retina. Although the lens of an adult's eye has a low transmittance to blue light, long-term exposure to blue light will cause degeneration of the retina and form photo retinitis. Therefore, in LED products, blue light leakage is absolutely not allowed.
In order to realize 4π light emitting, most products use a transparent substrate, so that light emitted by the LED chip is light emitted in 4π. The existing LED filament generally adopts a blue LED chip coated with a yellow phosphor to achieve the effect of emitting white light. However, the main component of the phosphor is generally silica gel, and the heat conductivity coefficient of the silica gel is only 0.2 W/(m·K). The generated heat is even hard to be dissipated. If the filament is not completely coated, since the substrate is transparent, the uncoated portion will leak blue light, causing damage to the human eye. On the other hand, in the case where the filament is completely coated, a large amount of heat generated by the LED chip is hard to be dissipated, and when the temperature exceeds a certain temperature in use, the silica gel cracks, which may also cause blue light leakage.
In addition to the problem of heat dissipation and blue light leakage, the existing LED filament lamp generally adopts a core column with a bracket, which is a glass bracket extending into the center of the bulb, and the LED filament is dispersed and fixed on the glass bracket. The LED filament is a fine and tiny industrial part product, and only automated machine production can maintain their consistency and reliability. Therefore, the production of a filament substrate, the mounting of pins and brackets, the die bonding of LED beads, the connection of gold wires, and the coating of phosphors should be automatically completed in an industrial automatic production machine. For an existing LED filament, one end is soldered to the upper end of a bracket, and the other end is soldered to the lower end of a core base. Soldering points are divided into upper and lower ends, and the existing LED filaments are generally in series or series-parallel connection to meet the voltage requirement, which requires more LED filaments, the soldering points are further increased, and the wick assembly is more complicated.
In summary, in order to further develop LED filament lamps, the prior art has yet to be further improved and developed.